linux-kernel - Re: [PATCH v2] memcg: reduce lock time at move charge (Was Re: [PATCH 04/10] memcg: disable local interrupts in lock_page

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Message-Id: <20101007215556.21412ae6.akpm@linux-foundation.org>
Date:	Thu, 7 Oct 2010 21:55:56 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Cc:	Daisuke Nishimura <nishimura@....nes.nec.co.jp>,
	Minchan Kim <minchan.kim@...il.com>,
	Greg Thelen <gthelen@...gle.com>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, containers@...ts.osdl.org,
	Andrea Righi <arighi@...eler.com>,
	Balbir Singh <balbir@...ux.vnet.ibm.com>
Subject: Re: [PATCH v2] memcg: reduce lock time at move charge (Was Re:
 [PATCH 04/10] memcg: disable local interrupts in lock_page_cgroup()

On Fri, 8 Oct 2010 13:37:12 +0900 KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com> wrote:

> On Thu, 7 Oct 2010 16:14:54 -0700
> Andrew Morton <akpm@...ux-foundation.org> wrote:
> 
> > On Thu, 7 Oct 2010 17:04:05 +0900
> > KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com> wrote:
> > 
> > > Now, at task migration among cgroup, memory cgroup scans page table and moving
> > > account if flags are properly set.
> > > 
> > > The core code, mem_cgroup_move_charge_pte_range() does
> > > 
> > >  	pte_offset_map_lock();
> > > 	for all ptes in a page table:
> > > 		1. look into page table, find_and_get a page
> > > 		2. remove it from LRU.
> > > 		3. move charge.
> > > 		4. putback to LRU. put_page()
> > > 	pte_offset_map_unlock();
> > > 
> > > for pte entries on a 3rd level? page table.
> > > 
> > > This pte_offset_map_lock seems a bit long. This patch modifies a rountine as
> > > 
> > > 	for 32 pages: pte_offset_map_lock()
> > > 		      find_and_get a page
> > > 		      record it
> > > 		      pte_offset_map_unlock()
> > > 	for all recorded pages
> > > 		      isolate it from LRU.
> > > 		      move charge
> > > 		      putback to LRU
> > > 	for all recorded pages
> > > 		      put_page()
> > 
> > The patch makes the code larger, more complex and slower!
> > 
> 
> Slower ?

Sure.  It walks the same data three times, potentially causing
thrashing in the L1 cache.  It takes and releases locks at a higher
frequency.  It increases the text size.

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